It is known to provide utility machinery with a number of belt and/or chain drives to power sub-systems for processing material handled by the machine or for operating other sub-systems of the machine. One example of the sort of utility machinery which is commonly fitted with multiple belt drives, is an agricultural crop gathering and processing vehicle, such as a self-propelled combine harvester or a forage harvester.
While a chain drive can offer high efficiency in power transmission, this form of drive tends to have quite a high maintenance requirement, e.g. in terms of chain lubrication. A more significant disadvantage is felt when a change in transmission ratio is required, under which circumstances it may prove necessary to fit one or more gearboxes into the drive line or to provide multiple sprocket sets and thereby add weight, cost and complexity.
In many cases, it has been found useful to use a belt drive rather than a chain. Belt drives comprise a belt that encompasses a driver pulley and a driven pulley. They usually run dry and changes in ratio can be achieved without necessarily using a gearbox. Changes in ratio may be achieved using a variable speed drive mechanism, for example in the form of a stepless transmission of the type often referred to as a continuously variable or infinitely variable transmission (CVT/IVT). In many belt drive transmissions of this general type, the ratio is varied by changing the distance between opposing members forming sheaves of a pulley. The sheaves are moveable axially relative to each other and this varies the pitch diameter and/or grip of a belt running between them. Commonly the sheaves of the driven pulley are biased towards each other by a resilient means and an actuator is used to control the relative position of the sheaves of the driven pulley.
If both sheaves move, the pitch or grip variation imparted to the running sides of the belt may be considered to be double acting. In many cases, however, one of the sheaves is positionally fixed axially and the other sheave moves axially in relation to it within the limits of a predetermined stroke. The length of the predetermined stroke and the angle of the sheave surface define the range of pitch or grip imparted to the belt. While the moveable sheave can be displaced axially with respect to the positionally fixed one, it is often fixed in respect of relative rotation. This is sometimes achieved by a splined connection between a hub carrying the fixed sheave and a sleeve carrying the moveable sheave.
In some known belt drive variators, the moveable sheave is biased towards the axially fixed sheave by means of a spring. One such arrangement is disclosed in U.S. Pat. No. 3,616,706 in which a through-bolt is used to pre-load the spring and exert a compressive force gripping the belt against slippage. One disadvantage of such an arrangement is that the casing holding the spring sticks out to one side. If the variator is to be positioned close to a frame, such as a side panel of a combine harvester, the spring casing may have be positioned outboard so as to provide preferred belt alignment. This will incur a width penalty equal to the depth of the spring casing. As the spring is pushing against the moveable sheave, this arrangement would therefore mean that the moveable sheave is also outboard and its associated control linkages may be both vulnerable to damage and may still further increase width or interfere with other mechanisms. It might in some cases be preferable to position the moveable sheave inboard and therefore facing the frame. Such an arrangement, however, is difficult to meet because the space available between the variator and the frame may not be wide enough to accommodate the spring housing or associated control linkages.
The torque, and hence the power a belt can transmit, is proportional to the axial load on the belt sides. If the load is too low, slippage and hence loss of power will occur. However, the load may not be increased infinitely, because the lifetime of a belt decreases dramatically if the axial load is too high. Hence there is a need for adjustment of the axial load on the sheaves in response to the transmitted power or torque.
A basic arrangement such as that proposed in U.S. Pat. No. 3,616,706 or GB-1555162 makes some provision for preventing problems of belt slip. The arrangement of GB-1555162 transmits power via the through-bolt that rotatably carries both sheaves. Splines assure a fixed rotational position of the sheaves relative to each other. The through-bolt transmits power via cam members adapted to push the opposing sheave halves forming the pulley towards each other in the event of increased power transmission. This tightens the grip on the sides of the belt that contact the two sheaves forming the pulley. It is a disadvantage of the proposal in GB-1555162 that this cam mechanism is external to the pulley, where it is exposed to environmental hazards such as dust or grease. This mechanism also operates by constantly pushing the moveable sheave against the fixed sheave, which may limit the life of some components such as thrust bearings.
When the speed of the driving pulley is decreased suddenly, the inertia of the driven components may temporarily drive the belt and the torque is reversed. The cam members lose contact and may be rotated a substantial distance from each other. When the speed of the driving pulley is increased again, the torque returns to its normal direction and the cam surfaces hit each other forcefully. Such backlash causes premature wear to the cams and their supports.
In addition, it may be noted that the sensitivity of the control mechanism at high belt pitch is the same as it is at low belt speeds and this may limit the changes in pitch and associated speed changes which can be made per increment in sheave separation.
Commonly the driven pulley is provided with a control mechanism for positively adjusting the distance between the driving pulley sheaves and hence the pitch of the belt. Such control mechanism is provided at the outside of the pulley, such that it enlarges the overall width of the drive variator and hence of the machinery it is mounted on.
In some known utility machinery, use of multiple belt drives can lead to increasing width as drives stack up one outside the other. This can lead to problems achieving overall width limits for road transport, e.g. three meters, whilst still providing a sufficiently wide frame for materials handling. One example of such a situation arises in agricultural vehicles and in particular some of those having a plurality of transverse belt and/or chain driven shafts arranged for tangential flow of harvested crop between, over or under drums, knives and similar processing equipment driven by those shafts. In EP-1044598, for example, the thresher part of a combine harvester is in the form of a multiple drum thresher operating in tangential flow with at least four drums. All the drums are connected in constant transmission ratios and are driven by a variable speed drive mechanism in the form of a speed variator. In this arrangement, however, fixed ratio driven wheels are positioned behind an engine driven input wheel and this might lead to complicated and lengthy ratio changes.
It is generally desirable in the art to develop systems which are compact and effective and to reduce one or more of the parts count, complexity and associated cost of providing, using and/or maintaining drive arrangements such as those used to drive components of a utility vehicle.